This is a clinical trial to determine whether a new FDA-approved oral drug (approved for rare metabolic diseases) could be used to treat CF patients. This drug, phenylbutyrate, may be able to restore normal chloride ion movements in body organs and glands. We would like to see whether high doses of this drug would be safe and work by restoring normal levels of chloride ion transport in a dose-dependent manner. We also would like to see how the CF patient absorbs, breaks down, and excretes the drug from the body. This is a Phase I dose escalatoin and safety study which means that we will enroll patients in the lowest dose first and then enroll additional people at higher doses into dose-limiting toxicity (serious side effects) is uncovered.